Hydrosilylation reactions, which entail the addition reaction of a Si—H functional compound with a compound having a carbon-carbon double bond or triple bond, are a useful means for synthesizing organosilicon compounds, and are synthesis reactions of industrial importance as well.
Platinum (Pt), palladium (Pd), and rhodium (Rh) compounds are known as catalysts for such hydrosilylation reactions. Of these, platinum compounds typified by Speier's catalyst and Karstedt's catalyst are most often used.
One problem with reactions in which platinum compounds are used as the catalyst is that, when adding a compound having Si—H functionality to a terminal olefin, internal rearrangement of the olefin arises as a secondary reaction. In such a system, because the catalyst does not exhibit addition reactivity with respect to internal olefins, unreacted olefin remains behind within the addition product. Therefore, in order to bring the reaction to completion, excess olefin which makes allowance for the portion that will be left behind due to secondary reactions must be used.
Another drawback is that, depending on the type of the olefin, selectivity of the α-adduct to the β-adduct worsens.
The largest problem is that the central metals Pt, Pd and Rh are all highly expensive noble metal elements. Numerous studies are being carried out in search of metal compound catalysts that are less expensive to use. In particular, although ruthenium (Ru) belongs to the noble metals, because it is a metal available at relatively low cost, there is a desire that it functions as a substitute for Pt, Pd and Rh.
In this connection, ruthenium compounds which have an η6-arene group and in which organopolysiloxane is bonded, or vinylsiloxane is coordinated, to a Ru metal center have been reported (Patent Document 1). Although these compounds have been shown to be effective in an addition reaction between methylhydrogenpolysiloxane and methylvinylpolysiloxane, the yield is low in a reaction at 120° C.; to obtain a high yield, reaction must be carried out at an elevated temperature of 160° C.
A number of patents relating to Ru catalysts are cited in Patent Document 1 as prior-art literature (Patent Documents 2 to 6), but none of these catalysts appear superior to noble metal element-based catalysts in terms of reactivity, selectivity or cost-effectiveness.
Olefin hydrogenation reactions are also industrially important reactions. Noble metals such as Pt, Pd and Rh are used in conventional catalysts for such reactions, and the use of ruthenium, which is inexpensive among the noble metals, is desired. For example, some such reactions have used trinuclear ruthenium complexes of the sort shown in Non-Patent Document 1, but further improvement is desired in terms of reaction temperature, yield and the like.
Non-Patent Document 2 discloses mononuclear ruthenium complexes that are effective in hydrogenation reactions on tetrasubstituted olefins, which are regarded as having a low reaction yield, but the turnover number is low and high-pressure reaction conditions are required.
Methods for reducing carbonyl compounds include methods that use an aluminum or boron hydride compound or use hydrogen in the presence of a noble metal catalyst. Of carbonyl compounds, with regard to the reduction of ketones and aldehydes, hydride reactants and noble metal catalysts for hydrogenation which can induce the reaction to proceed under mild conditions and are stable and easy to handle are known. However, to reduce carboxylic acid derivatives such as esters and amides, use is primarily made of methods that employ powerful reducing agents such as lithium aluminum hydride or borane (Non-Patent Document 3). Unfortunately, these reducing agents are ignitable, water-prohibitive substances, and thus difficult to handle. Moreover, even when removing the aluminum or boron compound from the target product following the reaction, care is needed in handling. In addition, high-temperature, high-pressure hydrogen is needed to reduce carboxylic acid derivatives.
Although numerous methods for the use of hydrosilane compounds and methylhydrogenpolysiloxanes, which are stable in air and easy to handle, as reducing agents have been reported, such reactions require the addition of a strong acid or Lewis acid and an expensive noble metal catalyst. Recently, carbonyl compound reduction reactions that use relatively inexpensive ruthenium as the catalyst have been reported. Some of these reports even mention examples of use in amide-reducing reactions that, in conventional methods, require harsh conditions. Examples of specific ruthenium catalysts are mentioned in Non-Patent Documents 4 to 7, although high-activity catalysts which exhibit a higher turnover number are desired.
The following mononuclear complex compounds having a bonds between ruthenium and silicon are known: divalent complexes having six-electron ligands (Non-Patent Documents 8 and 9), tetravalent complexes having two-electron ligands (Non-Patent Documents 10 and 11), tetravalent complexes having six-electron ligands (Non-Patent Documents 9 and 12), divalent complexes having thiourea groups on silicon (Non-Patent Document 13), divalent complexes having halogens on silicon (Non-Patent Document 14), anion complexes (Non-Patent Document 15), and divalent complexes having agostic Si—H ligands as two-electron ligands (Non-Patent Documents 15 and 16).
Divalent complexes which have no a bonds between ruthenium and silicon and which have agostic Si—H ligands as two-electron ligands are also known (Non-Patent Documents 16 and 17).
In addition, the following mononuclear complex compounds having isonitrile as two-electron ligands are known: divalent complexes which have two a bonds between ruthenium and silicon, have CO and also have halogen groups on silicon (Non-Patent Document 18), divalent complexes which both have two a bonds between ruthenium and silicon and also have CO (Non-Patent Document 19), divalent complexes which have one a bond between ruthenium and silicon and also have CO (Non-Patent Document 20), divalent complexes which have two a bonds between ruthenium and silicon and also have halogen groups on silicon (Non-Patent Document 21), divalent complexes which have only one a bond between ruthenium and silicon (Non-Patent Document 22), and divalent complexes which have no a bonds between ruthenium and silicon (Non-Patent Document 17).
However, above Non-Patent Documents 8 to 22 do not in any way suggest the possibility that the ruthenium complexes disclosed therein have catalytic activities in hydrosilylation reactions, olefin hydrogenation reactions and/or carbonyl compound reduction reactions.
As for examples of reactions in which a ruthenium complex is used as a catalyst, an addition reaction between ethylene and dimethylchlorosilane (Non-Patent Document 23) has been reported, but the reactivity and selectivity of this reaction is low.
Examples of addition reactions between disilane and ethylene (Non-Patent Document 24) have been reported, and a disilametallacycle structure as the intermediate has been proposed as the putative reaction mechanism. However, a ligand-containing complex structure has not been elucidated, and no identification has been made suggesting a dimetallacycle structure. The example reactions reported here also have a poor reactivity and selectivity, and cannot be regarded as complexes exhibiting adequate catalytic activity. Furthermore, the chief product of this reaction mechanism is vinylsilane or cyclic silane due to dehydrogenative silylation; only a trace amount of addition product is present.